1. Field of the Invention
The present invention relates to an organic electroluminescent element, a testing device thereof and a testing method thereof, and an organic electroluminescent display device. More specifically, the present invention relates to an organic electroluminescent element used as an electroluminescent element in an organic electroluminescent display, a testing device and a testing method each suitable for testing lifetime characteristics of an organic electroluminescent element, and an organic electroluminescent display device.
2. Description of the Related Art
Demands for a Flat Panel Display (FPD) which is a slim profile and low in power consumption and lightweight have recently increased with high advancement in information technology. Particularly, attention has been focused on organic electroluminescence (hereinafter, also referred to as “EL”) displays using organic luminescent materials because the displays can be driven at a low voltage and provide display with high brightness. Particularly, recent research and development remarkably improves luminescence efficiency of organic EL elements, and the organic EL displays including the organic EL elements have come into practical use.
Low-molecular organic EL materials and polymer organic EL materials may be mentioned as a luminescent material used in the organic EL elements. A general organic EL element using the polymer organic EL material has a structure, for example, in which, on a glass substrate, a transparent positive electrode made of indium tin oxide (hereinafter, also referred to as “ITO”), a positive hole transporting layer made of PEDOT/PSS (Poly(ethylene-dioxythiophene)/Poly(styrenesulfonate)), a luminescent layer made of a polymer organic EL material, a negative electrode made of Ca/Al, and the like are sequentially stacked. The organic EL element having such a structure is reported to attain a brightness of 10000 cd/m2 or more, a luminescence efficiency of several lm/W to dozens lm/w, and a lifetime of thousands to tens of thousands hours.
Such a conventional organic EL element has sufficiently high brightness and luminescence efficiency, but the lifetime is not enough for practical application as a commercial product. The application range has been limited. Therefore, various measures have been conventionally investigated in order to improve the lifetime characteristics of the organic EL element. For example, techniques regarding improvement in the luminance material itself (for example, referring to Patent Document 1 (Japanese Kohyo Publication No. Hei-11-508731 (pages 1 and 2) (corresponding to International Publication WO 97/40648 and U.S. Pat. No. 6,326,091)), or improvement in the negative electrode (for example, referring to Nonpatent Document 1 (Yong Cao, 3 others, “Ultrathin layer alkaline earth metals as stable electron-injecting electrodes for polymer light emitting diodes”, JOURNAL OF APPLIED PHYSICS, U.S., Sep. 15, 2000, vol. 88, No. 6, p. 3618)) have been proposed. However, even with these measures, the organic EL display has a lifetime much shorter than that of other FPDs using liquid crystal and the like, and therefore there is room for improvement in order to obtain an organic EL element which can be practically used in various applications.
No knowledge which is a clue to a relationship between a physical property value and the lifetime of the organic EL element has been obtained, and guideline for development in that what characteristics the element needs to have in order to improve the lifetime has not been found yet. In these respects, there is room for improvement.
Further, in a conventional lifetime test of organic EL elements, a method of performing aging test, in which an electric field is actually applied to an element to be tested to cause the element to produce luminescence, and thereby measuring a brightness half-life time of the element, is common. In such a testing method, the element used for the test is deteriorated and therefore can not be used as a commercial product after the test. The aging test needs a long time and therefore the working efficiency is very low. In addition, the organic EL element has a structure in which extremely thin film layers are stacked, and therefore variation in lifetime of the element is easily generated due to unevenness in film thickness of each layer, and it is difficult to determine the lifetime of the element from initial luminescence characteristics, and it is impossible that an element having a defect in lifetime characteristics is selected in the initial lighting test. Naturally, it is also impossible that elements are individually measured for lifetime characteristics. Therefore, in a conventional organic EL element-testing process, suppression of variation in the lifetime characteristics among the elements or selection of the element having a defect in the lifetime characteristics can not be performed sufficiently.
In development and improvement of materials of films constituting the organic EL element or production processes of the element, optimal conditions need to be found. Usually, conditions are optimized by subjecting an element prepared under certain conditions to lifetime test, and then subjecting an element prepared under another conditions to lifetime test based on a feedback from the previous lifetime test. In this case, it takes a very long time to find the optimal conditions if the elements prepared under each condition are subjected to the lifetime test using the conventional lifetime testing method of organic EL elements. Accordingly, also in this respect, it is expected that information on the lifetime characteristics of the element is obtained without performing the aging test.
Luminescence lifetime characteristics of the luminescent material used in the organic EL element recently have been widely used as a means for analyzing luminescence characteristics of the organic EL element, as seen in studies on temperature dependency of photoluminescence (hereinafter, also referred to as “PL”) intensity (for example, referring to Nonpatent Document 2 (Goushi Kenichi, 3 others, “Unusual luminescence characteristics of Ir(ppy)3 at low temperature”, Japan Society of Applied Physics Spring Meeting 2003, p. 1412 and 28p-A-4)) or change of PL characteristics in a stacked film layer (for example, referring to Nonpatent Document 3 (Goushi Kenichi, 3 others, “Exciton migration and energy transfer length of Ir(ppy)3” Japan Society of Applied Physics Autumn Meeting, 2003, p. 1206 and 1a-YL-7)) . This shows that the luminescence characteristics of the luminescent material have a great influence on the luminescence characteristics of the element and are used as an indicator for clarifying difference in luminescence characteristics between elements prepared under different conditions. However, there have been few cases of studying the PL characteristics of the organic EL element itself, and particularly, no cases of clarifying the relationship between the PL characteristics and the lifetime characteristics of the element has been reported yet.